The present invention relates to design of a magnetic recording head to minimize undershoots in readback pulses.
Magnetic recording heads are used to record and read information made up of alternating magnetization patterns on a magnetic recording medium. It is desirable to have the highest level of signal-to-noise ratio arising from the interaction between the recording head and medium. When reading recorded data from a recording medium, a higher signal-to-noise ratio takes the form of a desired central pulse achieving a higher amplitude relative to background interference (from neighboring pulses and other sources).
When a thin film magnetic recording head passes over a magnetic medium during the read operation, the generated waveform contains a leading and a trailing undershoot, in addition to the central pulse. These undershoots contain high frequency components which are comparable to the central pulse. If the central pulse were isolated, the magnitude of the undershoots would be approximately five percent to fifteen percent of the amplitude of the isolated pulse. These undershoots can lead to a forty percent reduction in the amplitude threshold margin of a central pulse and the pulse shape effects can reduce the window margin by fifty percent.
There are various known methods for making thin film recording heads. In one typical method, an insulating base layer of A1.sub.2 O.sub.3 is deposited on a substrate. One such substrate is known in the industry as ALSIMAG. Next, a seed layer, such as NiFe, is sputtered over the insulating base layer of A1.sub.2 O.sub.3. Photoresist is then spun over the seed layer and a pole piece pattern is formed in the photoresist by photolithographic techniques. After the resist is developed, pole material, such as NiFe, is deposited through the mask by means of electroplating.
After the plating of the first pole piece, a gap layer is deposited, such as a thin layer of A1.sub.2 O.sub.3. A coil structure with surrounding layers of insulation is also formed at the yoke of the pole structure. The second pole piece is next plated onto this structure. Normally, the width of the second pole at its tip (i.e. at the air bearing surface) is plated to be narrower than the width of the first pole at its tip. This confiquration is designed to avoid magnetic shorts occurring between the two pole pieces.
Both pole pieces are plated to be considerably wider than the final pole width desired. Thereafter, a thick resist is patterned on the second pole tip and over the yoke. The width of this pattern ultimately will determine the final width of the poles of the gap. Next, by a process known as ion milling, the excess magnetic material which has been plated along the width of the two pole tips is removed. In the normal process, the ion beam is impinged upon the head normal to the wafer surface during the first part of the milling process. This vertical milling leads to redeposition of magnetic material across the gap where it has been exposed during the milling process. This causes a magnetic short, which is removed by further milling.
The track trimming process as described above is ordinarily employed in the industry for obtaining uniform widths for both poles, where ninety degree walls are sought in the longitudinal direction. In practice, however, the process results in approximately 83.degree. wall angles. Such a deviation from the 90.degree. angle happens to reduce the magnitude of the undershoots from a recording head up to two percent. However, the attendant increase of the first pole in width at the gap leads to increased read/write fringing.
Undershoots can also be reduced by producing particularly thick poles--on the order of seven microns in thickness. Such reduction can amount to four percent in the size of the undershoots. However, this reduction does not outweigh the substantial increase in processing complexity to achieve such thick poles, particularly for poles made in the track trimming process. For example, inordinate trimming times would be required for the thicker poles.